1. Field of the Invention
The present invention relates generally to a slide exhaust brake system disposed midway of the exhaust pipe of a large-sized vehicle such as a track or bus for braking the vehicle running on a steep downhill by shutting off the exhaust gas passage of the exhaust pipe and, more particularly, to an improvement in a valve actuator for use with the slide exhaust brake system.
2. Description of the Prior Art
According to the prior art, the valve actuator of the above-specified type is constructed, as shown in FIG. 2, of a gas-tight cylinder casing 21 which in turn is composed of an elongated cylinder body 24 made of aluminum, iron or stainless steel and integrated at its upper end with a head wall 22 having an intake/exhaust port and at its lower end with a bottom wall 23 of aluminum with or without an intake/exhaust port. In the cylinder casing 21 thus composed, there is fitted a piston member 26 which clamps a packing 27. In the inner side of the bottom wall 23, there is anchored a spring bearing member 25 which is also made of aluminum. The piston member 26 has its piston rod 28 projecting to the outside through the bottom wall 23 and a partition 30. On the end portion of the projection of the piston rod 28, there is carried a plate valve member (although not shown) for opening or closing the exhaust gas passage of an exhaust brake system 31 which is fastened to both the bottom wall 23 and the partition 30. Incidentally, reference numeral 29 denotes a coil spring which is fitted under compression between the piston member 26 and the spring bearing member 25 for biasing the former member 26 upward. On the other hand, reference letters G and G' denote heat insulating gaskets which are sandwiched between the bearing member 25 and the bottom wall 23 and between the bottom wall 23 and the partition 30.
Generally speaking, the exhaust brake system is heated to a very high temperature while the vehicle is running because it is always exposed to the hot exhaust gas of the engine. The actuator has to be freed as much as possible from the thermal influences from the hot exhaust brake system so that it may operate smoothly and reliably. The counter-measures for this necessity have been desired. However, this desire cannot be fully satisfied by the actuator of the prior art, which has a tendency to fail to sufficiently reduce the heat to be transferred thereto in dependence upon the elongated integral structure of the cylinder body 24 of aluminum, iron or stainless steel, even with the provision of the heat insulating gaskets G and G'. That tendency causes the actuator to be still heated to a high temperature after a long run so that the piston member 26, especially its packing 27 is thermally influenced to lose its sealing characteristics with the cylinder body 24 at an early stage thereby to invite leakage. This results in a problem that the actuator has its piston function degraded to be troubled in its expected long use.
In order to provide a sliding face for the piston member, on the other hand, the elongated integral cylinder body 24 must have its whole inner circumference subjected to an anodizing, honing or chromium-plating treatment to have a hardened surface. This treatment raises the cost for the material and deteriorates the workability so that it accordingly raises the production cost.